The present invention relates to a composite polymeric material having high resistance to impact energy.
Specifically, the present invention relates to a composite polymeric material, to the corresponding preparation process and to manufactured articles having high impact energy absorption.
Currently, manufactured articles for automotive parts capable of absorbing impact energy, such as structures for vehicle interiors and exteriors, motorcycle crash helmets and others, are made of a single material having a polymeric base, such as expanded polystyrene, expanded polypropylene, or polyurethane foams.
These manufactured articles are generally produced with a molding technology which entails injecting an expandable polymer into a mold having a preset shape and then expanding the polymer in the presence of catalysts and/or expansion agents. At the end of the expansion, the polymer hardens and acquires the preset shape of the mold.
In particular, in the production of polyurethane manufactured articles, polyisocyanates and polyesters or glycols are injected into the mold and are made to polymerize in the presence of catalysts and expansion agents. During polymerization, the polyurethane foam forms and, by expanding, takes the shape of the mold.
However, manufactured articles resistant to impact energy made of a single polymeric material produced according to conventional technology are not free from having drawbacks in use, mainly due to the fact that they are made of a single polymeric material which does not provide an adequate resistance response to the impact, and to the shock wave. In particular, the only variable that is available to the designer is the density of the material.
For example, impact-resistant manufactured articles made of a single polymer according to the prior art, particularly expanded polystyrene helmets for motorcycles, have the drawback that they lose their impact-resistance characteristics after a single impact that causes a permanent deformation of the structure.
In the case of fenders for cars or of motorcycle helmets, permanent deformation after an impact accordingly compromises their safety characteristics, forcing the user to replace the manufactured article.
One of the aims of the present invention is to eliminate or substantially lessen the drawbacks of the prior art.
Another object of the present invention is to provide a composite polymeric material which has high-level impact energy resistance characteristics and has a long life.
A further object of the present invention is to provide a composite polymeric material which is simple to produce and does not entail high production costs.
Yet another object is to provide a manufactured article made of composite polymeric material which combines low weight with high mechanical strength characteristics.
Another object of the invention is to provide a method for producing manufactured articles made of an impact-resistant composite material whose execution entails modest operating and energy-related costs.
With the foregoing and other objects in view, there is provided, in accordance with a first aspect of the present invention, a composite material comprising a pre-expanded polymer, preferably in granular form, selected from the group consisting of polystyrene, polypropylene, a copolymer of polystyrene with a melting point above 90xc2x0 C. and mixtures thereof, dispersed in a resin selected from the group consisting of melaminic resin, phenolic resin, polyurethane resin and mixtures thereof, the polyurethane resin being the preferred one among them. The expression xe2x80x9cpolystyrene copolymerxe2x80x9d denotes a product of polymerization of polystyrene with another polymer or an alloy of polystyrene with another polymer.
Among polystyrene copolymers having a melting point above 90xc2x0 C., preference is given to those with a melting point above 100xc2x0 C.; among these, particular preference is given to the copolymer of polyphenylene oxide (PPO) and polystyrene (PS). By way of example, mention is made of the copolymer of polyphenylene oxide and polystyrene marketed under the trade-name NORYL EF (in granules or expandable beads) by the General Electric Co., USA, and of the product of the Huntsman Chemical Co., Chesapeake, Va., USA, marketed under the trade-name GECET (polymeric alloy of PS-PPO).
The pre-expanded polymer used in the present invention has advantageously been subjected beforehand to an expansion process and has a granular, spherical or bead-like shape.
The composite polymeric material according to the present invention preferably consists of a uniform dispersion of pre-expanded granules or beads in a matrix of a polyurethane, melaminic or phenolic resin.
Preferably, the polyurethane used in the present invention is obtained by condensation of an isocyanate or polyisocyanate with a compound possessing active hydrogen, both selected so as to have the following characteristics:
low reactivity; a delayed polymerization time of at least about 30 seconds allows the two components still in a fluid state to easily reach the lowest part of a mold wherein pre-expanded granules or beads are packed and then initiate the polymerization phase, so that when the polymerisation phase is completed, all (or almost all) the voids among the beads are filled with polyurethane (about 50% of the total volume of the mold);
optimal cohesion properties and adherence to the expanded beads;
optimal mechanical resistance;
low initial viscosity of the mixture of the two components, allowing an easy penetration in the voids created among the expanded beads;
high expansion capacity allowing the complete filling of the free spaces so as to obtain a homogeneous structure of the composite material;
high heat resistance;
high self-extinction.
Examples of such components are the isocyanate marketed by Dow Chemical Co., USA under the name VORACOR CD 526 and the polyol marketed by Dow Chemical Co., USA, under the name VORACOR CD 443.
The composite material having a polymeric base, according to the present invention, has high impact energy absorption because, following an impact, the impact energy is transmitted along the resinous structure, which has acquired a cellular structure whose cells internally contain the expanded polymeric granules, which undergo elastic deformation and considerably contribute to damping propagation of the shock wave and to ensuring the preservation of impact energy-absorbing properties even with subsequent impacts.
In view of these characteristics, the composite material according to the invention is particularly suitable to provide lightweight manufactured articles having high mechanical strength and resistance to impact energy and to repeated impacts.
Said manufactured articles comprise, in variable amounts, the composite material according to the invention, alone or associated with other impact-resistant materials.
Comparative tests have shown that the manufactured articles according to the invention have higher energy absorption than polystyrene and, for an equal density with respect to polyurethane, are more resistant to impact energy and to repeated impacts.
The manufactured articles according to the present invention can be entirely made of the above-described composite material or are constituted by multilayer structures in which, for example, one or more layers are made of the composite material and the remaining layers are made of other materials (polymeric and nonpolymeric).
According to another aspect of the present invention, said manufactured article is a liner for helmets, for example of the type for motorcycles, cars, work or sport. Helmet liners manufactured with the composite polymeric material according to the invention are highly resistant to impact energy and in particular maintain their impact-resistance characteristics substantially unchanged even after one or more impacts.
In particular, it has been observed that the permanent deformation of the surface structure of the manufactured article according to the invention produced by a medium-energy impact does not compromise, except to a limited extent, its strength and safety characteristics, making them suitable for prolonged use.
According to a further aspect, the present invention relates to a life preserver which comprises the composite material described above. Advantageously, said life preserver is constituted by a core made of very hard polymeric material or wood or steel, covered with a layer of the composite material according to the invention. The life preserver according to the invention has a weight which can be likened to that of life preservers of the prior art together with high impact resistance.
In accordance with yet another aspect of the present invention, said manufactured article is an automotive interior, such as door paddings, door panes, knee bolsters, head restraint systems, sun vizors, instrument panels, rear parcel shelves, or an automotive exterior such as bumpers, body supports or an automotive cavity filling.
In accordance with another aspect of the present invention, a method for producing manufactured articles made of composite material is provided, said method comprising the steps of:
injecting into a mold a pre-expanded polymer selected from the group consisting of polystyrene, polypropylene, polystyrene copolymer with a melting point above 90xc2x0 C. and mixtures thereof, and in then injecting a resin or the reagents that polymerize into a resin selected from the group consisting of polyurethane resin, phenolic resin, melaminic resin and mixtures thereof;
expanding said resin inside the mold, dispersing said pre-expanded polymer therein.
Preferably, the pre-expanded polymer is supplied in the form of granules or beads which have dimensions between 3 and 10 mm and the polymerization reaction to generate said resin occurs at a temperature between 70 and 140xc2x0 C.
According to a preferred embodiment of the method according to the invention, the following operating steps are performed in sequence:
filling a mold with granules of a pre-expanded polymer selected from the group consisting of polystyrene, polypropylene, copolymer of polyphenylene oxide and polystyrene and mixtures thereof;
subsequently injecting in the mold an isocyanate or a polyisocyanate and a compound possessing active hydrogen thereby they are filled in void spaces among the granules,
condensing said isocyanate or polyisocyanate with a compound possessing active hydrogen to generate, by polymerizing, a polyurethane foam which, by expanding, substantially completely fills the spaces among the pre-expanded polymer granules.
It has been observed that by initially filling the mold with the pre-expanded polymer in granular or bead form and then injecting the polyurethane, it is possible to determine the weight and therefore the density of the manufactured article.
Preferably, the reaction for condensation between the isocyanate and the compound possessing active hydrogen determines a temperature between 70 and 140xc2x0 C. In the execution of the method it is possible to use expansion agents, additives and catalysts for the condensation reaction capable of accelerating polymerization and resin hardening times.
The reagents are preferably injected by using positive-displacement pumps with a variable rotation rate and a mixer for mixing the two streams of reagent.
The term xe2x80x9ccompound possessing active hydrogenxe2x80x9d denotes a compound that may be replaced by sodium and with a few compounds having hydrogen atoms not readily replaced by sodium. In the reaction with said compounds possessing active hydrogen, the hydrogen atom becomes attached to the nitrogen of the isocyanate and the remainder of the active-hydrogen compound becomes attached to the carbonyl carbon. Said compounds possessing active hydrogen include compounds having amino, hydroxyl, carboxyl groups, particularly polyesters, polyethers and polyols, the last being preferred.
The polyurethane condensation or foaming step advantageously entails expansion times lasting between 10 and 15 minutes, during which the polyurethane foam expands into the empty volume between the polymer granules.
In accordance with an embodiment of the method according to the invention, the mold is filled initially with the pre-expanded polymer.
Preferably, the mold is initially completely filled with beads of expanded material with suitable dimensions and bulk density.
The polyurethane resin suitable for use in the present invention has a high fluidity and a delayed polymerization time, of at least 30 seconds, so as to allow the passage of the resin through all the expanded material filled in the mold, in the void spaces among the beads.
Moreover, the polyurethane resin used in the present invention has a high polymerization heat.
Due to the properties of the isocyanate and active hydrogen containing compound, as indicated above, in particular due to their high fluidity and low polymerization time, the isocyanate and the active hydrogen containing compound are filled in substantially all the empty spaces among the pre-expanded polymer beads.
Due to the granular (or bead-like) shape of the polymer, empty spaces remain between the granules and are gradually filled during the expansion of the polyurethane resin. A composite material is thus obtained which has a substantially uniform distribution of the pre-expanded polymer granules.
It has been observed that the pre-expanded polymer granules or beads, during the exothermic reaction for the polymerization of the polyurethane resins, behave differently according to the chemical nature of the polymer being used.
In particular, when granules or beads of expanded polypropylene or of an interpolymer of polyphenylene oxide and expanded polystyrene are used, the polyurethane condensation reaction can occur at a temperature between 100 and 140xc2x0 C. without causing their melting or breakdown. The polypropylene beads, amalgamated in the polyurethane foam, by being filled with air, can compress under the impact energy and then resume their initial position, thus dissipating the kinetic energy. It has been observed that the resulting composite material has the highest impact-resistance characteristics.
Instead, when polystyrene granules or beads are used in the process, the generation of heat (T=80-100xc2x0 C.) typical of the exothermic polymerization reaction of polyurethane causes them to melt. The granules, under the heat, melt inside the polyurethane foam, creating voids.
The granules or beads used in the process according to the invention are expandable polymers, expanded for example by subjecting polymeric expandable beads to a pre-expansion in a pressurized vessel using dry saturated steam. Then the pre-expanded beads are subjected to a drying step and to a maturing stage to compensate for the temporary vacuum in the cell structure after pre-expansion. This is achieved, for example, by blowing the prefoam into an air-permeable silo. After sufficient air has been diffused into the cells, the matured beads are stable and ready for processing.
The matured prefoam is blown into a mold of the desired shape, preferably until the mold is almost completely filled.
The pre-expanded granules used in the invention preferably have a density between 10 and 80 g/l, while the resins used preferably have a density between 20 and 400 g/l.
The method according to the present invention provides manufactured articles made of composite material with modest operating and energy-related costs. In particular, operating costs are reduced thanks to the possibility of using resin molds which are commonly commercially available at low cost. Molds used in the field of the invention are preferably thermostat-controlled, so as to maintain a temperature, during the resin polymerization step, that is lower than the melting temperature of the foamed polymer described above.
Energy expenditure is reduced due to the low operating temperatures of the processing plant.
According to an embodiment of the method according to the invention, manufactured articles of preset shape, such as for example outer shells, reinforcement elements, hooks, nets etcetera, are placed beforehand in the molds.